1. Field of the Invention
This invention concerns a lower end shield assembly of a magnetron. The assembly can be mounted in a microwave oven, sometimes referred to as an electronic range, to generate microwaves. The lower end shield is of an improved construction and is capable of reducing dark current. Dark current is current that does not contribute to the oscillation of the magnetron.
2. Description of the Prior Art
Generally, a magnetron is a type of diode which is used for efficiently producing microwaves. A magnetron is usually mounted to a microwave oven which is used for heating and thawing of food items. As shown in FIG. 1 of the accompanying drawings, a magnetron for a microwave oven comprises an anode cylinder 1 provided on its inner periphery with a plurality of radially extending vanes 3, and a direct-heated filament (referred to as a cathode) 2 disposed axially in a centered relation within the anode cylinder.
In addition, the magnetron includes a magnetic circuit comprising upper and lower magnets 5, 6, upper and lower plates 7, 8, and upper and lower magnetic poles 9, 10. An output section comprises an antenna 11, an antenna ceramic 12, an exhaust pipe 13 and an antenna cap 14.
When the filament 2 is energized, thermions are emitted from the filament into an active space 4, which is defined between the filament 2 and the vanes 3 of the anode cylinder. The emitted thermions effect cycloidal movement by virtue of an electric field induced between the filament and the vanes and a magnetic flux applied within the space by the magnetic circuit, thereby applying energy to the vanes to generate microwaves. Microwaves are then emitted through the output section to the exterior of the magnetron and radiated into the cavity of the microwave oven via a waveguide to heat or thaw the food items placed within the oven.
A filter circuit comprising a through type condenser 19, a first case 20, a second case 21 and a choke coil 23, and disposed adjacent to an input section, acts to prevent certain higher harmonics and fundamental waves generated in the anode cylinder 1 from leaking to the exterior of the magnetron via the filament 2, a center lead 15, a side lead 17 and the input section. To prevent thermions emitted from the filament 2 from escaping out of the active space 4, as shown in FIG. 2, which is an enlargement of area A in FIG. 1, an upper end shield 16 in the form of a frustrum of a circular cone and a lower end shield 18 in the form of a cylinder are brazed to the upper ends of the center and side leads 15, 17, respectively. The upper and lower shields substantially block the upper and lower ends of the active space. To prevent breakage of the filament 2 by an external shock, a spacer 22 made of an insulating material is joined to the center and side leads 15, 17 passing therethrough.
The microwave energy generated by the thermions which effect cycloidal movement in the active space is transferred to the vanes 3. However, some of the thermions escape through the gaps between the upper and lower edges of the vanes 3 and the upper and lower end shields 16, 18 without applying energy to the vanes. Thus, dark current, which is unable to contribute to oscillation of the magnetron, is produced.
Since there is a potential difference of about 4 KV between the upper and lower magnetic poles 9, 10 and the upper and lower end shields 16, 18, the end shield must be positioned exactly in a centered relation within the central openings of the magnetic poles. If uniform clearances between them are not maintained, a spark discharge may occur and cause breakage of the magnetron.
The prior art cathode assembly of the magnetron for the microwave oven is optimally manufactured to have the upper and lower end shields assembled to be exactly centered with respect to the central axis of the anode cylinder, thereby minimizing leakage of thermions out of the active space. However, in practice the upper and lower end shields, which provid support for the filament, are assembled as if they are in a floating condition, as shown in FIG. 2. This means that the upper and lower shields may not be exactly centered Within the magnetic poles, but located eccentrically. As a result, thermions emitted from the filament may escape through wider portions of the gaps between the upper and lower edges of the vanes and the upper and lower end shields, thereby lowering the efficiency of the magnetron.
Since the distances between the upper and lower magnetic poles and the upper and lower end shields, at which a potential difference of 4 KV is provided, may be narrowed to generate a spark, a complicated, troublesome adjustment process of exactly centering the upper and lower end shields about the anode cylinder is required. This process typically entails adjusting the center and side leads in each direction with a jig.